Piezoelectric crystal units with malleable terminals and epoxy-filler sealant

ABSTRACT

Crystal plate mounting means integral with malleable terminals of a piezoelectric crystal unit accommodate differently dimensioned crystal plates and permit spacing of the terminals for registry with circuit board perforations without stressloading such plates. Other means integral with the terminals isolate the crystal plate from stress-loading when the free ends of the terminals are stressed. The illustrated mounting means comprise bifurcations formed in one end of each terminal and the illustrated other means comprise a paddle section of each terminal embedded in a resilient organic adhesive securing and hermetically sealing each terminal to an eyelet. The malleable terminals are solderable and readily deformable to secure registry between the free ends of the terminals and perforations in printed circuit boards. The organic sealant is compatible with the malleable terminal material and withstands stresses induced therein when the terminals are stressed and when the eyelet is cold welded to an envelope. The disclosed method includes the steps of securing a pair of terminals to an eyelet, adapting the ends of the terminals to support a crystal plate without stressloading such crystal plate, securing a crystal plate in a stress free condition to the terminals, and cold welding the eyelet to an envelope by a cold weld processing step to avoid mass-loading the crystal plate.

United States Patent [191 Scott,'Jr. et al.

[451 Nov. 19, 1974 PIEZOELECTRIC CRYSTAL UNITS WITH MALLEABLE TERMINALSAND EPOXY-FILLER SEALANT [75] Inventors: Kelley E. Scott, Jr., Plano;Daryl M.

Kemper, Sandwich, both of 111.; Lloyd E. Grove, Geneva; Ronald J. Kiess,Decatur, both of 1nd.

[73] Assignee: CTS Corporation, Elkhart, Ind.

[22] Filed: Feb. 15, 1972 [21] Appl. No.: 226,430

Related US. Application Data [62] Division of Ser. No. 830,956, June 6,1969, Pat. No.

[52] US. Cl 310/9.l, 310/8.9, 310/94 [51] Int. Cl I101v 7/00 [58] Fieldof Search 310/89, 9.1, 9.4;

[56] References Cited UNITED STATES PATENTS 2,399,919 5/1946 Garrison310/9.4 2,434,903 1/1948 Bokovoy et a1... 310/9.4 2,457,145 12/1948 Gray310/9.4 2,513,870 7/1950 Hoffman 310/94 X 2,597,797 5/1952 Holmbeck310/9.1 X 2,676,275 4/1954 Bigler 3lO/9.4 X 2,785,321 3/1957 lmler310/9.1 3,017,525 l/1962 Wolfskill 310/9.4 3,022,431 2/1962 McKnight310/9.4

Primary Examiner-Mark O. Budd Attorney, Agent, or Firm-John J. Gaydos 57] ABSTRACT Crystal plate mounting means integral with malleableterminals of a piezoelectric crystal unit accommodate differentlydimensioned crystal plates and permit spacing of the terminals forregistry with circuit board perforations without stress-loading suchplates. Other means integral with the terminals isolate the crystalplate from stress-loading when the free ends of the terminals arestressed. The illustrated mounting means comprise bifurcations formed inone end of each terminal and the illustrated other means comprise apaddle section of each terminal embedded in a resilient organic adhesivesecuring and hermetically sealing each terminal to an eyelet. Themalleable terminals are solderable and readily deformable to secureregistry between the free ends of the terminals and perforations inprinted circuit boards. The organic sealant is compatible with themalleable terminal material and withstands stresses induced therein whenthe terminals are stressed and when the eyelet is cold welded to anenvelope. The disclosed method includes the steps of securing a pair ofterminals to an eyelet, adapting the ends of the terminals to support acrystal plate without stress-loading such crystal plate, securing acrystal plate in a stress free condition to the terminals, and coldwelding the eyelet to an envelope by a cold weld processing step toavoid mass-loading the crystal plate.

15 Claims, 7 Drawing Figures PATENTEU sv 1 91914 3, 849.681

SHEET 10F 2 FIGURE 2 PIEZOELECTRIC CRYSTAL UNITS WITH MALLEABLETERMINALS AND EPOXY-FILLER SEALANT This is a division of applicationSer. No. 830,956 filed June 6, 1969 now US. Pat. No. 3,656,217 issued onApr. 18, 1972.

This invention relates to piezoelectric crystal units and, moreparticularly, to an improved construction of such units and to a methodof making the same.

The satisfactory use of crystal units in frequency control circuits ispredicated on the long term accuracy and frequency stability of suchunits. It will be appreciated that even when it is permissable for acrystal unit to exhibit a change in operating frequency of 3 X parts perday, it is necessary to prevent mass-loading of the crystal plate, i.e.,alteration of the resonant characteristics of a crystal plate because ofthe deposition of contaminants such as water vapor, dust, or otherorganic or inorganic materials on one or more surfaces of the crystalplate. In addition to avoiding mass-loading, it is desirable to avoidstress-loading of the crystal plate, i.e., alteration of the resonantcharacteristics of a crystal plate because of mechanical stressesapplied to the crystal plate by the means that are used to mount thecrystal plate in an enclosure or envelope.

Because of the importance of operating a crystal plate in acontaminant-free environment, elaborate and expensive techniques havebeen used in the past to hermetically seal one or more crystal plateswithin a contaminant-free enclosure. This expedient has also beenutilized in order to avoid the frequency shift problems that can arisedue to the occurrence of chemical reactions involving the electrodematerial deposited on the crystal plate. Prior art sealing techniqueshave comprised the steps of hermetically sealing a pair of terminals inan eyelet to form a header, mounting a crystal plate on the header, andhermetically sealing the header to a metal or glass envelope with thecrystal plate disposed within such envelope.

Heretofore, headers have been characterized either as matched glass" orcompression glass" headers. In

a matched glass header, a single material is used to fabricate theterminals and eyelets; and a vitreous material, having the same thermalcoefficient of expansion as the eyelet and terminals, seals theterminals to the eyelet. Normally, the eyelet and terminals are made ofKovar, and the vitreous material is a glass that has been selected tohave a thermal coefficient of expansion substantially the same as thatof Kovar. The sealing process is accomplished by placing molten orliquid glass in the eyelet around the terminals and then cooling theglass to cause it to shrink and tightly grip the terminals, and becomevitreous. Thereafter, the similar coefficients of thermal expansion ofthe Kovar and vitreous glass ensure that the seal between the terminalsand eyelet will be maintained. Compression glass headers are made bygenerally following the same process steps used to make matched glassheaders. However, the vitreous glass is selected to have a coefficientof thermal expansion greater than the terminal material, and the eyeletmaterial differs from the terminal material and is selected to have acoefficient of thermal expansion greater than the vitreous glass. In theprocessing of these headers, the glass continues to shrink around theterminals after it becomes vitreous whereas the eyelet shrinks aroundthe vitreous glass as the header is cooled.

In both matched glass and compression glass headers, the integrity ofthe terminal to eyelet seal depends upon the attainment of a good sealbetween the terminal and glass. In turn, this seal is dependent at leastpartly upon the presence of a tenacious oxide coating on the portion ofthe terminal embedded within the glass. As will be understood by thoseskilled in the art. the requirement for a tenacious oxide coating on theterminal and the necessary exposure of the terminal to a molten glassinherently precludes the use of many potential terminal materialsbecause of the extreme corrosiveness of molten glass and the ease withwhich many metallic oxides will dissolve in molten glass. This is truein fact. even when glasses having a melting point of as low as 450 C. to700 C. are used as the vitreous sealant. More specifically, thecorrosiveness of molten glass has generally precluded the use oftenninal materials such as copper or tin coated copper. Accordingly, itwould be desirable to provide a new and improved hermetically sealedcrystal unit wherein a sealant and fabricating process is used thatwould permit the use of terminals fabricated from copper, tin coatedcopper, or other easily soldered electrically conductive malleablematerials.

The integrity of conventional terminal to eyelet seals also depends,among other things, on the change in coefficient of thermal expansionthat is exhibited as a molten glass is cooled from a liquid andsupercooled liquid state to a vitreous state. As is well known topersons skilled in the glass art, any given glass is characterized by arelatively constant coefiicient of thermal expansion while the glass isvitreous and below the transformation range temperatures of such glass.At temperatures above the transformation range, i.e., normally aboveabout 600 C., the coefficient of thermal expansion of a given glassincreases by a factor of 2 or 3. In practice, this means that when avitreous glass with a coefficient of thermal expansion equal to Kovar isheated to a liquid state, i.e., to a temperature of l,200 C. to l,500C., and then allowed to cool, the glass will shrink or contract two tothree times as much, per unit measure, as Kovar until a fictivetemperature of the glass is reached. Then, the glass becomes vitreousand in theory contracts at the same rate as Kovar. Since the increasedrate of contraction takes place over a range of 300 to 600 Centrigradeafter the glass has macroscopically become a solid, it will beappreciated that the increased contraction of the glass results in acompression type seal even between such glass and Kovar and thiscompression inevitably creates stresses in the vitreous sealant that inturn make such sealant relatively susceptible to crazing or cracking. Inaddition, the observable fictive temperature of a glass varies as afunction of the cooling rate of the glass and the actual compressiveforces exerted by a vitreous glass cannot be precisely predicted unlessthe cooling rate of the glass is precisely controlled.

The relative ease with which vitreous materials may be cracked or brokenduring handling places limitations on many of the process steps that maybe practiced during the manufacture of crystal units. For example, onlya minimum amount of stress may be applied to a header when securingtogether the header and envelope in order to avoid damaging the vitreoussealant and thereby destroying the hermetic seal between the terminalsand eyelet. Accordingly, it would be desirable to provide a new andimproved hermetically sealed crystal unit wherein the header may besubjected to relatively great stresses during a manufacturing processwithout damaging the hermetic seal. The known hermetic sealants are alsosusceptible to damage while being installed in electrical equipmentmaking use of standard printed circuit boards. For example, slightdeviations in the relative positions of the crystal unit terminals andcircuit board perforations can prevent registry of the terminals withsuch apertures. In such cases, the relatively stiff terminals must bebent or otherwise deformed with the concomitant risk of stressing thevitreous sealant and destroying the hermetic sea].

It is desirable to precisely locate crystal unit terminals so that theywill register with apertures in printed circuit boards, and it is alsodesirable to very precisely locate the terminals so that they cansupport the crystal plate. In some applications, the required spacingbetween terminals for proper registry with a printed circuit boardprevents optimum terminal spacing from the viewpoint of supporting thecrystal plate. In these situations, it is necessary to space theterminals for registry with the circuit board and attach to theterminals separate crystal plate mounting means that resiliently gripthe crystal plate and facilitate the completion of a solder connectionbetween the mounting means and electrodes on the crystal plate. Inaddition to increasing the cost of a crystal unit, this arrangement isobjectionable because the mounting means apply a compressive force andstress-load the crystal plate. It therefore would be desirable toprovide improved crystal plate mounting means that accommodate variouslysized crystal plates without stress-loading such crystal plates.

When separate crystal plate mounting means are spot welded or otherwisesecured to the ends of terminals it is necessary to specifically orientthe terminals within the eyelet so as to maintain proper orientation ofthe mounting means relative to the eyelet while a hermetic sealent isapplied to secure the terminal to the eyelet. When separate mountingmeans have been secured to the terminals after the hermetic seal hasbeen completed, it has still been necessary to maintain a preciseorientation of the mounting means relative to the eyelet while securingthe mounting means to the terminals. Accordingly, it would be desirableto provide an improved mounting means and method of manufacturing acrystal unit that eliminates the necessity of maintaining the criticalorientation of mounting means relative to an eyelet while making ahermetic seal between the terminals and such eyelet or while securingthe mounting means to the terminals.

In contemporary crystal units, metal eyelets are soldered or brazed tometal envelopes. The use of these high temperature techniques increasesthe risk of massloading as the result of vapors forming a deposit on acrystal plate within an envelope. The fact that metallic vapors may forma deposit on a crystal plate and thereby cause a change in frequency iswell known and described in the Klingspom U.S. Pat. No. 3,028,262, datedApr. 3, 1962, and entitled Method For The Frequency Tuning OfPiezoelectric Oscillators". It will thus be appreciated that any meansused for hermetically sealing an eyelet to a metallic envelope thatinvolves the use of heat or molten materials can potentially result inmass-loading a crystal plate within the envelope. Because of theseproblems, attempts have been made to secure metal eyelets to a metalenvelope by means of apparatus that do not involve the use of heat ormolten materials. These attempts have involved the use of apparatus andequipment of the type described in Sowter U.S. Pat. No. 2,522,408entitled Cold Pressure Welding". In practice, however, these effortshave not been completely satisfactory because the stresses created inthe eyelet during the cold welding process can easily damage thevitreous sealant material in the eyelet. Accordingly, it would bedesirable to provide an improved crystal unit wherein a metal eyelet iscold welded to a metal envelope without damaging the hermetic sealbetween a pair of terminals and the eyelet.

Accordingly, it is an object of the present invention to provide a newand improved piezoelectric crystal unit. Another object of the presentinvention is to provide a new and improved method of makingpiezoelectric crystal units. A further object of the present inventionis to provide a new and improved hermetically sealed crystal unitwherein the hermetic sealant accommodates and permits the use ofterminals made from a readily solderable malleable material. Anadditional object of the present invention is to provide a new andimproved crystal unit that is capable of withstanding manufacturing andhandling stresses without damage to hermetic seals associated therewith.Yet another object of the present invention is to provide a new andimproved means for mounting a crystal plate that does not stress-loadsuch plate. Yet a further object of the present invention is to providea new and improved means for mounting a crystal plate that dispenseswith the need for separate structural elements attached to the terminalsof the crystal unit and that will accommodate variously dimensionedcrystal plates. Yet an additioal object of the present invention is toprovide a new and improved crystal unit wherein the terminals thereofmay be deformed and stressed during assembly and subsequent handlingwithout stress-loading the crystal plate and without damaging a hermeticseal between the terminals and eyelet during such deformation. Stillanother object of the present invention is to provide a new and improvedcrystal unit incorporating a sealant means that is compatible withmalleable and readily solderable terminals. Still a further object ofthe present invention is to provide an improved crystal unit thatfacilitates the assembly of a pair of terminals and an eyelet withoutregard to the orientation of crystal plate mounting means relative tothe eyelet. A more specific object of the present invention is toprovide a new and improved method of manufacturing a crystal unitwherein a crystal plate is supported directly by a pair of terminals. Astill more specific object of the present invention is to provide a newand improved crystal unit wherein an eyelet is hermetically sealed to anenvelope without mass-loading a crystal plate within the envelope. Aneven more specific object of the present invention is to provide a newand improved crystal unit having a resilient hermetic sealant capable ofwithstanding stresses induced therein. These and other objects andadvantages of the present invention will become apparent as thefollowing description proceeds, and the features of noveltycharacterizing the invention will be pointed out with particularity inthe claims annexed to and forming a pan of this specification.

The present invention is concerned with a piezoelectn'c crystal unitthat preferably is hennetically sealed. Crystal units embodying thepresent invention include an envelope, a header comprising an eyelet andterminals, and a crystal plate supported by mounting means integral withthe terminals. The mounting means accommodate differently dimensionedcrystal plates and permit spacing of the terminals for registry with acircuit board without stress-loading such plates. Other means, integralwith the terminals, isolate the crystal plates from stress-loading whenthe free ends of the terminals are bent or otherwise deformed. In theillustrated embodiment, the mounting means comprise bifurcations thatare cut, abraded, or otherwise formed in one end of each terminal.Preferably, the mounting means are formed after the terminals have beenassembled with an eyelet in order to avoid maintaining preciseorientation of the mounting means relative to each other during assemblyof the terminals and eyelet. The exemplified other means integral withthe terminals for isolating the crystal plate from stress-loadingcomprise a paddle section of each terminal that is embedded in the meansused to secure the terminals to the eyelet. Preferably, the terminalsare made from a malleable material such as aluminum, copper, ortin-coated copper. Other aspects of the invention are concerned withusing a cold-welding process to secure together the header and envelopeand with using a resilient nonvitreous material such as an organicadhesive material to secure and hermetically seal the terminals to theeyelet. The non-vitreous sealant is compatible with malleable terminalmaterials and maintains the integrity of a hermetic seal when the headerand envelope are coldwelded.

For a better understanding of the present invention, reference may behad to the accompanying drawings wherein the same reference numeralshave been applied to like parts and wherein:

FIG. 1 is an isometric view of a piezoelectric crystal unit embodyingfeatures of the present invention;

FIG. 2 is an exploded isometric view of the crystal unit illustrated inFIG. 1;

FIG. 3 is a cross-sectional view taken along the lines III-III in FIG.1;

FIG. 4 is a cross-sectional view taken along the lines IV-IV in FIG. 3,assuming that the crystal unit in FIG. 3 is shown in full;

FIG. 5 is a graph showing the relationship between the specific volumeand temperature of a vitreous material and a non-vitreous material;

FIG. 6 is a view similar to FIG. 4 illustrating another embodiment ofthe invention; and

FIG. 7 is a view similar to FIGS. 4 and 5 illustrating still anotherembodiment of the invention.

Referring now to the drawings, and more particularly to FIG. 1, acrystal unit 10 embodying the present invention comprises an envelope11, a pair of terminals l2, l3, and ari eyelet 14 comprising anapertured base, a continuous sidewall connected to the base, and aflange extending from a peripheral edge of the sidewall. Means forsupporting the terminals on the eyelet include a hermetic sealant thatcomprises a resilient organic adhesive 16 which maintains a hermeticseal be tween the terminals and eyelet. As best illustrated in FIGS. 3and 4, a crystal plate 17 is supported by mounting means that form anintegral part of the terminals l2, l3. Deposits of a conductive adhesivematerial such as solder or epoxy mechanically secure and electricallyconnect the mounting means with a pair of conventional crystal plateelectrodes 19, 21. As best illustrated in FIGS. 2 and 3, the mountingmeans comprise bifurcated portions 22, 23 of the terminals and when thecrystal plate 17 is positioned on such bifurcated portions, deposits ofconductive epoxy 24 fixedly secure the crystal plate 17 to thebifurcations 22a, 22b. 23a. and 23b. The bifurcations on each terminalpreferably are spaced apart a distance slightly greater than thethickness of the thickest crystal plate expected to be supported by theterminals 12, 13 so that a crystal plate may be readily slipped intoplace between the bifurcations on each terminal without beingstress-loaded. In FIG. 2, the tenninal 12 has been rotated slightly tobetter illustrate this spacing. When being mounted on the terminals 12,13, the crystal plate is positioned without constraint between thebifurcations, and conductive epoxy 24 is deposited to secure the crystalplate to the bifurcations. After the epoxy 24 has cured and becomerelatively rigid, the crystal plate 17 is firmly supported by theterminals 12, 13 and yet remains in a relaxed or unstressed condition.Although it is normally preferred to form each of the mounting meanswith two bifurcations as illustrated because of the ease with which thecrystal plate 17 is assembled therewith, the mounting means may beembodied in other forms. For example, one of the bifurcations may beremoved from each terminal so that each mounting means comprises asingle bifurcation disposed against a face of the crystal plate and aledge or shoulder at the base of such single bifurcation for supportingthe bottom edge of the crystal plate, i.e., the edge of the crystalplate adjacent to the eyelet 14. When it is desired to form the mountingmeans from a single bifurcation, such bifurcation may be fabricated bybending and shaping the terminal to form a shoulder and bifurcation, byswaging the end of the terminal to form a shoulder and bifurcation, orby removing one of the pair of illustrated bifurcations from theillustrated terminals 12, 13. When the mounting means are constructedaccording to any of the above teachings, at least one bifurcation orcrystal plate mounting portion of each of the terminals will be disposedadjacent to a face of the crystal plate.

Preferably, the mounting means are not formed until after the eyelet andterminals have been assembled to gether to form a header. When a sealantis used to hermetically seal a pair of terminals in an eyelet, it isonly necessary to place the terminals in the apertures of the eyelet,axially position the terminals relative to the eyelet, and maintain therelative positions of these elements until the sealant, whether vitreousor nonvitreous, becomes sufficiently rigid to maintain the terminals inassembled relation with the eyelet. With particular reference to theillustrated embodiment, it will be noted that since the terminals l2, 13are not bifurcated prior to being secured to the eyelet 16, it is notnecessary to orient the terminals with regard to the crystal platemounting means during the above described steps. Any suitable means maybe used for forming the bifurcations 22, 23 and such means includes butis not limited to apparatus such as a saw. By forming the mounting meansafter the eyelet and terminal is assembled, it will be appreciated thatsuch means may be formed in exact alignment for receiving a crystalplate without stress-loading such plate. Furthermore, this procedureeliminates the necessity of orienting separate crystal plate mountingmeans that are assembled with the terminals. It should now be apparentthat the process of forming crystal plate mounting means integral withthe terminals after fabrication of a header is also useful even whensuch header comprises one or more terminals insert molded or otherwisesecured to the eyelet made of a plastic or similar material.

When the terminals 12, 13 are supported in nonvitreous means, such as anorganic adhesive material as illustrated, it is preferable that theterminals each be provided with stress isolating means for insulatingthe mounting means from stresses caused by bending, twisting, orotherwise stressing the free ends of the terminals, e.g., ends 12a and13a. In the crystal unit 10, such stress isolating means include paddlesections 12b and 1312 which are formed by swaging the terminals toprovide debilitated segments between the ends thereof. In manyapplications, it normally would not be expected that a torque wouldbeapplied to the free ends 120, 13a of the terminals. However, when aterminal is inadvertently bent to the dotted line position of terminal12 in the manner illustrated in FIG. 4, forces directed along a linegenerally perpendicular to the plane of the drawing and applied to thefree end 12a can break the hermetic seal around the tenninal l2 andstress-load the crystal plate 17. Actual tests have been made on twodifferent header and crystal plate assemblies to illustrate theusefulness of the paddle sections 12b and 13b. One of these assemblies,herein referred to as assembly A", corresponded to the constructionillustrated in FIG. 4 and the other assembly, herein referred to asassembly B, similarly corresponded except that the terminalcorresponding to terminal 12 was not provided with any stress isolatingmeans. After the terminals of assembly A were bent to the dotted lineposition of tenninal 12 in FIG. 4, forces applied to the free ends ofthe tenninals caused them to turn about an axis defined by the solidline position of terminal 12 in FIG. 4. Continued application of suchforces caused the terminals to actually twist apart at the paddlesections, but at no time during the test was there any indi cation thatthe portion of the terminal between the paddle sections and mountingmeans moved relative to the sealant or that the hermetic seal was brokenbetween such portions and the sealant, and at no time during the testwas there any indication that a torque was transmitted to the mountingmeans. However, when a force was applied to the bent terminal 12 ofassembly B, the body of the terminal started to turn in the sealant, themounting means started to turn, and the crystal plate was mechanicallystressed. When continued force was applied, the mounting means actuallystarted to turn or rotate about an axis defined by the solid lineposition of terminal 12 in FIG. 4. As this occurred, the entire portionof the terminal embedded within the sealant started to turn relative tothe sealant, thus breaking the hermetic seal therebetween, and themounting means applied a stress to the crystal plate and actuallyfractured the corner of the crystal plate to which it was attached. Itwill be noted that the portions of the terminals l2, l3 embedded withinthe sealant 16 are substantially straight, i.e., the portions of theterminals above and below the debilitated sections are in line with eachother. Therefore, the terminals 12, 13 may each be randomly orientedabout the longitudinal axis thereof and have been positioned as shown inthe drawings only for clarity of illustration. In summary, the terminalsl2, 13 comprise straight or linear portions randomly oriented within thesealant 16, and the paddle sections 12b, 13b formed in such portionsprevent the transmission of stresses from the free ends 12a, I3a of theterminals to the mounting means 22, 23. In addition, the sections 12b,13b prevent the seal between the terminals and sealant 16 from beingbroken along the portions of the terminals between the mounting meansand sections 12b, 13b even when the sealant between the sealant andremaining portions of the terminals is destroyed.

When the terminals l2, 13 are stressed, the organic adhesive sealant 16will not normally be cracked or chipped because it is relativelyresilient. Preferably, the organic adhesive 16 is an epoxy material.Since epoxy will readily adhere to a wide range of materials that may beused to fabricate the eyelet l4 and terminals l2, 13, the specificcoefficient of thermal expansion of such material is not as critical asit otherwise would be if the sealant were relatively non-adhesivevitreous material. In the illustrated embodiment of the invention theeyelet 14 is made of aluminum and the terminals 12, 13 are made of tincoated copper and the sealant 16 is a conventional epoxy resin. Whenusing this material the terminals l2, 13 are positioned in the apertures26, 27 of the eyelet 14 as best indicated by FIGS. 3 and 4, and theuncured epoxy is dispensed into the eyelet 14 around the terminals. Thenthe eyelet, terminals, and sealant are heated to approximately 177 C. inorder to cure the epoxy and form a solidified but relatively resilientsealant around the terminals l2, 13. The adhesive 16 adheres to theterminals and eyelet and maintains a hermetic seal therebetween; and, inorder to reduce the strain placed on such seal during thermal cycling,the thermal coefficient of expansion of the epoxy 16 is modified so thatit will approximately equal the thermal coefficient of expansion of thealuminum eyelet l4, i.e., about 25 X 10' per degree Centigrade. Theepoxy l6 readily adheres to the eyelet and terminals, and is relativelyresilient and not readily cracked or crazed. Some of the otheradvantages attained by the use of the epoxy will be better understoodand more readily explained by comparing some of the characteristics of atypical prior known vitreous sealant and the epoxy 16.

Accordingly, reference is now made to the graph of FIG. 5 which showsthe relationship between the specific volume (cubic centimeters pergram) and temperature (degrees Centigrade) of these two materials.Curves C, D, E, and F illustrate the manner in which specific volumevaries as a function of temperature of a vitrifiable glass sealant. Oncurve C, point G is representative of a typical melting point of theglass material and at temperatures above T the glass is liquid while attemperatures between T and T the glass is a supercooled liquid. Thecurves D, E, and F illustrate the relationship between specific volumeand temperature of the glass after it has become vitreous, and whetherthe specific volume follows curve D, E, or F depends on the rate atwhich the glass is cooled. If the glass is cooled relatively fast, itwill have a fictive temperature of T and a specific volume along curve Dwhereas slow cooling results in a fictive temperature of T and aspecific volume along curve F. The curve E and fictive temperature Tcorrespond to one of an infinite number of cooling rates intermediatethe two cooling rates corresponding to curves D and F. As will beunderstood, the temperature range T to T represents the transformationrange of the glass and vitrification of the supercooled liquid glassoccurs at a temperature within this transformation range. The curves Hand K approximately represent the relationships between specific volumeand temperature of the epoxy 16, with the curve H illustrating theconstant temperature volumetric change that occurs as the epoxy curesand the curve K illustrating the volumetric change of the cured epoxy asthe temperature thereof is reduced to room temperature from a curingtemperature T of about 177 C.

The slopes of the various curves shown in FIG. are approximatelyindicative of the coefficients of thermal expansion of the glass andepoxy. With reference to the curves C, D, E, and F, it should be notedthat when the molten glass is deposited in an eyelet, it will cool fromthe melting point T (from 900 to 1,100 C.) and exhibit a relativelyconstant coefficient of thermal expansion as a fictive temperature isapproached. Then, depending upon the rate of cooling of the supercooledliquid glass, a fictive temperature between T and T will be reached andthe glass will become vitreous. With further cooling the thermalcoefficient of expansion will be reduced to from /:3 to /2 of thecoefficient of thermal expansion of the supercooled liquid glass.

It now should be apparent that when a vitreous material is used as asealant, the eyelet and terminals must be subjected to a much highertemperature, i.e., T than is the case when an organic adhesive materialis used which cures at a substantially lower temperature, i.e., T Inaddition, even though the rate of contraction or shrinkage (the slope ofcurves D, E, F) of a vitreous material may be predicted with a highdegree of certainty, the actual percent change in specific volume, i.e.,the actual shrinkage of the vitreous material, is relativelyunpredictable unless very rigid control is maintained over cooling ratessince the actual shrinkage of the vitreous material is dependent uponthe rate of cooling and the fictive temperature at which the supercooledliquid glass becomes vitreous. Therefore, the actual strength of ahennetic seal that depends solely on the actual amount of shrinkage of avitreous material is not precisely predictable. However, in the case ofan epoxy, the actual shrinkage of the material after curing orsolidification is realtively predictable and essentially independent ofthe cooling rate. In addition,

the actual amount of shrinkage or volumetric change of g the epoxy isnot as critical as in the case of glass because the seal is aided by theadhesive qualities of the epoxy and does not depend solely on thecompressive action of the sealant in the same manner as a vitreous glassseal. Other advantages of using an organic adhesive sealant should alsonow be readily apparent. For example, the tin coating on the copperterminals remains intact when the sealant 16 is used, whereas the tincoating would be quickly removed if it were exposed to molten glass.

The coefficient of thermal expansion of the epoxy sealant 16 isapproximately equal to that of aluminum, i.e., about 25 X per degreeCentigrade and the coefficient of thermal expansion of the vitreousglass typified by curves D-F is about 1 l X 10 per degree Centigrade.This difference in coefflcients is indicated by the different slopes ofcurves D-F and curve K, it of course being understood that the actualcoefficient of thermal expansion of the vitreous material and epoxy l6actually is a nonlinear function of temperature and that suchcoefficients have been treated as a linear function of temperaturesolely for the purpose of illustration in FIG. 5. However, thecoefficient of thermal expansion may, for practical purposes, beconsidered to be fairly uniform over a relatively narrow temperaturerange and thus it will be appreciated that the relatively narrow rangeof processing temperatures required for an epoxy sealant provides theadvantage that the total volumetric change of the epoxy is generallymore predictable than the total volumetric change of a vitreous materialthat must be processed over a relatively wide temperature range.

For the purposes of this application, the term epoxy material is meantto refer to the class of organic adhesive materials characterized by amolecular structure that includes a three member ring consisting of anoxygen atom attached to two adjacent carbon atoms and this term is meantto include catalysts, curing agents, and filler materials, whetherorganic or inorganic, that are used to extend or modify variousproperties of such material. In the exemplified construction, the epoxyresin was mixed with an aromatio polyamine based catalyst. Thepreviously mentioned material that was used to modify the coefficient ofthermal expansion comprised silica powder passable through a standard325 mesh sieve and 40 parts by weight of such filler were added to sixtyparts by weight of the resin-catalyst mixture.

The present invention alleviates the problems of processing a sealant atextremely high temperatures and the inherent difficulties that areencountered in such processing, including the necessity of exercisingrigid control over temperatures and cooling rates of materials. Inaddition, the eyelet and terminals may now be fabricated from a wideselection of materials. As one example, the terminals may now be made ofa malleable electrically conductive material, i.e., a material havingthe characteristics of copper or aluminum. The prior art techniques haveplaced limitations on the materials that could be used to fabricateeyelets because, among other things, of limitations imposed by therelatively low thermal coefiicients of expansion of available vitreoussealants and the corrosive action of molten glass sealants. Theselimitations have necessitated the use of relatively expensive alloymaterials such as Kovar and other metal alloys. An extremely well suitedinexpensive eyelet material is aluminum and even though some vitreousmaterials have been heretofore proclaimed as usable for coatingaluminum, such vitreous materials have in fact had thermal coefficientsof expansion of only about 16.4 X 10* per degree Centigrade. Since thepresent disclosure teaches how to construct a crystal unit incorporatingan aluminum eyelet and a sealant that can be made to very closely matchthe thermal expansion characteristics of aluminum, it will beappreciated that the present invention constitutes a substantial stepforward in the art.

After completion of the steps of forming the bifurcations 22, 23 andsecuring the crystal plate 17 to the bifurcations, the flange 29 of theeyelet 14 is pressure or cold welded to the flange 31 of the aluminumenvelope 11. During this step, high temperatures are avoided so that thecrystal plate 17 will not be exposed to metallic or organic vaporcontaminants that could mass-load the crystal plate. The stressesapplied to the header and sealant 16 during this step do not damage thehermetic seal between the terminals l2, l3 and eyelet 14 because thesealant 16 is sufficiently resilient to withstand such stresses withoutcrazing or cracking. The cold weld between the flanges 29 and 31 isaccomplished by placing the flanges between a pair of dies and applyinga sufficient amount of pressure to opposite sides of the overlappedflanges to cause the material in the flanges to flow and weld together.When this pressure is applied, the flanges 29, 31 are deformed andassume the cross-sectional configuration illustrated in FIGS. 3 and 4.Prior to cold welding the flanges 29, 31, the surfaces to be weldedshould be suitably cleaned, and during the cold welding process thecross-section or thickness of the overlapping flanges 29, 31 arepreferably reduced approximately 70 percent along the weld line. Detailsof suitable cleaning and welding techniques are described in thepreviously identified Sowter patent and such description is specificallyincorporated herein by reference.

The ease with which different sizes of crystal plates may be supportedon a single size of header or on a pair of terminals spaced for properregistry with a printed circuit board will be best understood by havinga reference to FIGS. 6 and 7 wherein the envelopes of the illustratedcrystal units have been omitted for purposes of clarity. In FIG. 6, theeyelet 32 and sealant 36 are substantially identical to the eyelet 14and sealant 16 of the crystal unit 10. In addition, the crystal platemounting ends 37a, 38a, of the terminals 37, 38 as well as the portionsof those terminals embedded within the sealant 36 are substantiallyidentical to the corresponding portions of the terminals l2, 13.However, the crystal plate 39 is physically smaller than the crystalplate 17. In order to accommodate this smaller crystal plate, theterminals 37, 38 have been formed with the bifurcated portions 40, 41thereof directed toward each other so as to support the crystal plate 39without mechanically stressing such crystal plate.

The embodiment illustrated in FIG. 7 differs from the embodiment of FIG.6 only in that the bifurcated portions 44, 46 of the terminals 47, 48are directed away from each other in order to support the crystal plate49. As can be seen from a comparison of FIG. 7 and FIG. 4, the crystalplate 49 is physically larger than the crystal plate 17. In otherrespects, the embodiment of FIG. 7 is the same as the embodiment of FIG.4 with the eyelet 51 being substantially identical to the eyelet l4 andthe sealant 52 being substantially identical to the sealant 16. Althoughit would be possible to form the bifurcated portions of the terminals37, 38 and 47, 48 prior to the time that such terminals are hermeticallysealed in the eyelets, it is preferable to assemble nonbifurcatedstraight terminals with the eyelets without regard to the orientation ofthe terminals and then bifurcate the ends of the terminals and bend orotherwise form the crystal mounting plate ends of the terminals 37, 38,47, 48 to accommodate the particular crystal plate that is to be mountedthereon. In the embodiments of FIGS. 6 and 7, as in the embodiment ofFIG. 4, the bottom of the crystal plate is nestedly supported by themounting means with at least one bifurcation of the mounting meansdisposed adjacent to a face of the crystal plate. In all threeembodiments of the invention, bifurcations define the ends of a pair ofterminals adjacent to a crystal plate within an envelope. As best shownin FIGS. 3 and 4, the peripheral bottom edge of the crystal plate is atleast partially bounded by the ledge or shoulder at the base of thebifurcations, but the peripheral side edges of the crystal plate are notcovered or enclosed by the mounting means. Each bifurcation on eachterminal defines one side of a slot or crystal plate receiving portalthat pennits unrestrained lateral movement of the crystal plate. Thisstructural arrangement facilitates the mounting of differently sizedcrystal plates on a given pair of mounting means and also ensures thatthe crystal plate will not be stressloaded as a result of the mountingmeans engaging the peripheral side edges thereof.

the he foregoing description of the various exemplifications of theinvention, it will be apparent that there is disclosed herein a new andimproved piezoelectric crystal unit and method for making the same thatovercome the aforementioned problems and disadvantages in the art andthat accomplish the stated objects of the present invention.

While there has been illustrated and described herein what is at presentconsidered to be preferred embodiments of the present invention and apreferred method of manufacturing a crystal unit, it will be appreciatedthat numerous changes and modifications are likely to occur to thoseskilled in the art, and it is intended in the appended claims to coverall those changes and modifications which fall within the true spiritand scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

l. A piezoelectric crystal unit comprising an eyelet having a pair ofterminal receiving apertures, a pair of conductive malleable terminalssupported in the apertures of the eyelet, each of the tenninals having afree end extending from the eyelet, a crystal plate having a pair offaces, an envelope surrounding the crystal plate and secured to theeyelet, and means formed integrally with the terminals for mounting thecrystal plate in a substantially stress-free condition, said meanscomprising a bifurcated portion on each of said pair of tenninalsdisposed adjacent to a face of said crystal plate, each of thebifurcated portions being provided with a slot defined by a pair of armsand a bottom edge, the crystal plate being disposed in and supported bythe slot by the pair of arms and the bottom edge of the slot wherebystress-loading of the crystal plate by the mounting means is prevented.

2. The piezoelectric crystal unit of claim 1 wherein the eyelet includesa flange, the envelope includes a flange overlapping the flange of theeyelet, and the flanges are cold welded together thereby to secureassembly of the eyelet with the envelope.

3. The piezoelectric crystal unit of claim 1 wherein a sealant securesthe terminals to the eyelet and comprises an organic adhesive materialmixed with a filler material.

4. The piezoelectric crystal unit of claim 3 wherein said organicadhesive material comprises a material that includes a three-member ringconsisting of an oxygen atom attached to two different carbon atoms andwherein such material adhesively secures the terminals to the eyelet.

5. The piezoelectric crystal unit of claim 4 wherein the organicadhesive material has a coefficient of thermal expansion substantiallythe same as the coefficient of thermal expansion of the eyelet.

6. The piezoelectric crystal unit of claim 1 wherein the crystal platemounting means define a portal opening upwardly away from the eyeletthereby to accommodate variously dimensioned crystal plates in astressfree condition.

7. The piezoelectric crystal unit of claim 1 wherein said terminals areprovided with stress-isolating means intermediate the free ends thereofand the means for mounting the crystal plate, whereby stresses in thefree ends of the terminals are isolated from the means for mounting thecrystal plate.

8. The piezoelectric crystal unit of claim 7 wherein thestress-isolating means comprise a paddle section on each of theterminals.

9. A piezoelectric crystal unit comprising a metal eyelet, a pair ofterminals extending through the eyelet, means secured to the eyeletsupporting the terminals, mounting means on the terminals, a crystalplate supported on said mounting means, and an envelope secured to theeyelet, said means secured to the eyelet supporting the terminalscomprising an epoxy material mixed with a filler material having acoefficient of thermal expansion substantially the same as that of themetal eyelet.

10. The piezoelectric crystal unit of claim 9 wherein said mountingmeans comprise a bifurcated portion on each of the terminals disposedadjacent to a face of the crystal plate.

11. The piezoelectric crystal unit of claim 9 wherein the terminalsinclude a straight segment embedded in the epoxy material for isolatingstresses from the mounting means.

12. The piezoelectric crystal unit of claim 11 wherein the means forisolating stresses comprise a debilitated paddle section in the straightsegment embedded in the organic material.

13. A piezoelectric crystal unit comprising an eyelet, a pair ofterminals each having a section secured to the eyelet and a free endprojecting away from the eyelet, mounting means for supporting a crystalplate on the terminals, a crystal plate supported on said mountingmeans, an envelope secured to the eyelet, and means for isolatingstresses in the free ends of the terminals from the mounting meanswhereby forces applied to the free ends of the terminals are isolatedfrom the mounting means, said means for isolating stresses comprising adebilitated segment of each terminal.

14. The piezoelectric crystal unit of claim 13 wherein a resilientsealant secures the terminals to the eyelet and the debilitated segmentof each-terminal is embedded in said sealant.

15. The piezoelectric crystal unit of claim 14 wherein said debilitatedsegments comprise a swaged section of the terminals.

1. A PIEZOELECTRIC CRYSTAL UNIT COMPRISING AN EYELEY HAVING A PAIR OFTERMINAL RECEIVING APERTURES, A PAIR OF CONDUCTIVE MALLEABLE TERMINALSSUPPORTED IN THE APERTURES OF THE EYELET, EACH OF THE TERMINALS HAVING AFREE END EXTENDING FROM THE EYELET, A CRYSTAL PLATE HAVING A PAIR OFFACES, AN ENVELOPE SURROUNDING THE CRYSTAL PLATE AND SECURED TO THEEYELET, AND MEANS FORMED INTEGRALLY WITH THE TERMINALS FOR MOUNTING THECRYSTAL PLATE IS SUBSTANTIALLY STRESS-FREE CONDITION, SAID MEANSCOMPRISING A BIFURCATED PORTION ON EACH OF SAID PAIR OF TERMINALSDISPOSED ADJACENT TO A FACE OF SAID CRYSTAL PLATE, EACH OF THEBIFURCATED PORTIONS BEING PROVIDED WITH A SLOT DEFINED BY A PAIR OF ARMSAND A BOTTOM EDGE, THE CRYSTAL PLATE BEING DISPOSED IN AND SUPPORTED BYTHE SLOT BY THE PAIR OF ARMS AND THE BOTTOM EDGE OF THE SLOT WHEREBYSTRESS-LOADING OF THE CRYSTAL PLATE BY THE MOUNTING MEANS IS PREVENTED.2. The piezoelectric crystal unit of claim 1 wherein the eyelet includesa flange, the envelope includes a flange overlapping the flange of theeyelet, and the flanges are cold welded together thereby to secureassembly of the eyelet with the envelope.
 3. The piezoelectric crystalunit of claim 1 wherein a sealant secures the terminals to the eyeletand comprises an organic adhesive material mixed with a filler material.4. The piezoelectric crystal unit of claim 3 wherein said organicadhesive material comprises a material that includes a three-member ringconsisting of an oxygen atom attached to two different carbon atoms andwherein such material adhesively secures the terminals to the eyelet. 5.The piezoelectric crystal unit of claim 4 wherein the organic adhesivematerial has a coefficient of thermal expansion substantially the sameas the coefficient of thermal expansion of the eyelet.
 6. Thepiezoelectric crystal unit of claim 1 wherein the crystal plate mountingmeans define a portal opening upwardly away from the eyelet thereby toaccommodate variously dimensioned crystal plates in a stress-freecondition.
 7. The piezoelectric crystal unit of claim 1 wherein saidterminals are provided with stress-isolating means intermediate the freeends thereof and the means for mounting the crystal plate, wherebystresses in the free ends of the terminals are isolated from the meansfor mounting the crystal plate.
 8. The piezoelectric crystal unit ofclaim 7 wherein the stress-isolating means comprise a paddle section oneach of the terminals.
 9. A piezoelectric crystal unit comprising ametal eyelet, a pair of terminals extending through the eyelet, meanssecured to the eyelet supporting the terminals, mounting means on theterminals, a crystal plate supported on said mounting means, and anenvelope secured to the eyelet, said means secured to the eyeletsupporting the terminals comprising an epoxy material mixed with afiller material having a coefficient of thermal expansion substantiallythe same as that of the metal eyelet.
 10. The piezoelectric crystal unitof claim 9 wherein said mounting means comprise a bifurcated portion oneach of the terminals disposed adjacent to a face of the crystal plate.11. The piezoelectric crystal unit of claim 9 wherein the terminalsinclude a straight segment embedded in the epoxy material for isolatingstresses from the mounting means.
 12. The piezoelectric crystal unit ofclaim 11 wherein the means for isolating stresses comprise a debilitatedpaddle section in the straight segment embedded in the organic material.13. A piezoelectric crystal unit comprising an eyelet, a pair ofterminals each having a section secured to the eyelet and a free endprojecting away from the eyelet, mounting means for supporting a crystalplate on the terminals, a crystal plate supported on said mountingmeans, an envelope secured to the eyelet, and means for isolatingstresses in the free ends of the terminals from the mounting meanswhereby forces applied to the free ends of the terminals are isolatedfrom the mounting means, said means for isolating stresses comprising adebilitated segment of each terminal.
 14. The piezoelectric crystal unitof claim 13 wherein a resilient sealant secures the terminals to theeyelet and the debilitated segment of each terminal is embedded in saidsealant.
 15. The piezoelectric crystal unit of claim 14 wherein saiddebilitated segments comprise a swaged section of the terminals.